Cycles displacement: banding despite full float .exr texture
I'm pretty sure that all you did was save the noise with more precision.  I think if you blurred it enough to get rid of the noise then you will have nothing but a wavy cloud of a displacement.

Try to make your depth map using blender modeling/render tools, and then use that result for Displace.  

Here's a set-up that I found on a BlenderArtists.org thread that should help you along.  If you have any questions please ask.  I'll update my answer tomorrow with a bit more detail on how this works.  I just thought I could get you started with this little bit for now.

Once you have the setup right, you can just do a camera render and save it as an .exr, that will be your high-precision displacement map.